1. Field of the Invention
The present invention relates to an electronic apparatus having key input means and, more particularly, to an electronic apparatus for executing a predetermined information processing by a central processing unit (CPU) which can stop the oscillation of operation clocks and can shift to a stand-by mode for a key input waiting period of time.
2. Related Background Art
Hitherto, in electronic apparatuses which are driven by a battery or the like, there has been known a technique to execute a clock control for a key input waiting period of time in order to reduce a current consumption of the apparatus.
For instance, there has been known a construction such that operation clocks of a CPU are stopped for the key input waiting period of time and the operation clocks are supplied to the CPU for a predetermined period by a timer interruption and a key input is read or a construction such that two oscillators to generate high and low frequency clocks are provided and the CPU is made operative by the low frequency clocks for a key input waiting period of time.
According to the former conventional technique, even if a timer interruption of a period of about 10 msec (milliseconds) is executed, a time of at least 5 msec is needed for warming-up of the CPU clocks and for reading of a key input and a current consumption can be merely reduced to about the half.
On the other hand, according to the latter conventional technique, a current consumption which can be saved depends on frequency characteristics of the current consumption of a memory and, particularly, in the case of the recent ROM of a large capacity, even if the operation clock frequency of the CPU is reduced, the current consumption cannot be largely reduced.
Therefore, there is also considered a method whereby an RAM whose current consumption is proportional to the operation clocks is used and only the contents of key input processings are previously transferred to the RAM and a key input waiting processing is performed in the RAM. However, the above method has problems such that the data in the RAM becomes random (changes) due to electrostatic noises, the CPU rans away, there is a tendency such that as the capacity of the RAM is large, a current consumption increases, a current consumption remarkably changes due to the maker of the device, and the like.